This invention relates generally to a device for projecting reshaping a beam of radiation and an imaging lidar system incorporating this device. More particularly, this invention relates to a new and improved laser light beam homogenizer which transforms a laser beam with spatially inhomogeneous intensity into a beam with a more nearly spatially uniform intensity pattern.
There is currently a need for discrete devices which expand, reshape and project beams of radiation. An example of an application which may require such a device is the imaging lidar system disclosed in U.S. Pat. No. 4,862,257 (assigned to the assignee hereof and incorporated herein by reference) wherein a pulsed light source (laser) projects typically circular pulses of light at a target in a backscattering medium with the reflected light pulses being detected by one or more gated cameras. In certain situations, it may be advantageous to reshape the pulsed light from the original circular cross-section shape to another configuration, generally rectangular or square. This need is particularly important in the imaging lidar system described in U.S. patent application Ser. No. 565,631 filed Aug. 10, 1990, now U.S. Pat. No. 5,231,401 which is also assigned to the assignee hereof and incorporated herein by reference. Presently, it is difficult to effectively and accurately expand, reshape and project radiation beams such as laser beams.
Laser beam homogenizers are well known in the art. For example, U.S. Pat. No. 4,744,615 to Fan et al (all of the contents of which are incorporated herein by reference). discloses a laser beam homogenizer employing a light tunnel and is described below. As is well known, in order for a light tunnel to have a reasonable geometry (i.e., dimensions), the input laser beam to the light tunnel should have a significant beam divergence angle (herein defined as the angle between the most divergent marginal ray in the light tunnel and the beam axis). In accordance with the laser beam homogenizer of U.S. Pat. No. 4,744,615, a lens focuses a laser beam onto a focal point S, which defines a focal plane perpendicular to the optical axis of the laser beam and thereby creates a diverging laser beam with a significant beam divergence angle .theta.. The light tunnel receives most of the diverging laser light.
The entrance of the light tunnel is a square aperture which limits the entering light to a square cross-section and defines the marginal rays of the light in the tunnel. The length L of the tunnel is defined herein for purposes of illustration as a length extending all the way to the focal plane, even when the physical length of the tunnel is less. This is done because a tunnel extending all the way to the focal plane is optically equivalent to a tunnel which extends forward toward the source even farther; or one which does not extend even to the focal plane, so long as the marginal rays in the light tunnel are not changed thereby.
The beam divergence angle .theta. is defined as the angle between the axis and the most divergent marginal ray in the light tunnel. Actually, there is a most divergent marginal ray with respect to each of the reflective sides of the tunnel. With a square tunnel coaxial with the axis, the most divergent marginal ray striking each reflective side of the tunnel strikes the inside front edge of each side at the midpoint of the reflective side, and the beam divergence angle is the same with respect to each of the sides.
The light tunnel has a length and width such that the diverging laser light portion reflected from the top side and the diverging laser light portion reflected from the bottom side each exactly fills the exit face of the light tunnel. A central portion of the diverging laser light passes through the tunnel without any reflection, while the peripheral portions are reflected.
Since the rays in each of the reflected portions of the light are still diverging after reflection, the reflected rays may be extended backwards to define virtual focal points or virtual light sources. Actually, two additional virtual focal points or sources are formed by the light which is reflected once from the left and right sides of the light tunnel; and four additional virtual focal points or sources are formed by the light which makes a reflection from each of two adjacent sides of each of the four corners of the tunnel.